1. Field of the Invention
The present invention relates to an engine cooling structure, and more particularly, to an engine cooling structure suitable for cooling a portable engine having a cylinder provided with an exhaust port, and an exhaust muffler connected to the exhaust port, and suitable for driving a backpack blower, a backpack power applicator or a hand held bush cutter.
2. Description of the Related Art
A portable engine for various work machines such as mentioned above is requested to cool its cylinder and exhaust muffler and to discharge an exhaust gas of the lowest possible temperature. Various previously proposed engine cooling structures are devised to use cooling air efficiently by properly controlling the flow of cooling air.
FIGS. 7 and 8 show a cooling structure disclosed in JP-A No. 2002-303148 applied to a portable engine for driving a backpack blower A shown in FIG. 6. Referring to FIG. 6, the backpack blower A includes an engine E, a blower 1, a back frame 15 holding the engine E and the blower 1, a flexible pipe 16 connected to the discharge pipe of the blower 1, a duct 17, and a nozzle 18. An operator supports the back frame 15 holding the engine E and the blower 1 on the operator's shoulders by straps 19 connected to the back frame 15. Air is blown through the nozzle 18 to blow and collect dead leaves M and trash.
Referring to FIG. 7 showing the blower 1 and the engine E in a schematic plan view, the blower 1 has a fan 2 and a fan casing 3 housing the fan 2. The engine E for driving the fan 2 has a cylinder 8 and is joined to a back part of the fan casing 3. The cylinder 8 of the engine E is disposed behind the fan casing 2 opposite to a cooling air inlet opening 10 formed in the back wall 3a of the fan casing 3. An exhaust muffler 11 is connected to the right side of the cylinder 8, and a carburetor 12 and an air cleaner 13 are connected in that order to the left side of the cylinder 8.
Spaces extending over the cylinder 8, and the left and the back side of the cylinder 8 are covered with a cylinder cover 100 formed by processing a metal sheet. Spaces extending over the exhaust muffler 11, and the right and the front side of the exhaust muffler 11 are covered with muffler covers 101 and 102. A shroud 103 is disposed outside the muffler cover 101 as indicated by imaginary lines in FIG. 7.
As shown in FIG. 8, the exhaust muffler 11 is provided in it's lower end part with an exhaust gas discharge opening 110.
In the portable engine E shown in FIG. 7, cooling air is taken from the fan casing 3 through the cooling air inlet opening 10 into the space around the cylinder 8. The cooling air flows into a space around the exhaust muffler 11 after cooling the cylinder 8 to cool the exhaust muffler 11. Then, the cooling air is discharged backward together with the exhaust gas through an opening formed in the shroud 103 shown in FIG. 7.
The cooling structure shown in FIGS. 7 and 8 can efficiently cool the cylinder 8 and the exhaust muffler 11. However, the cooling air is used first for cooling the cylinder 8, and flows through the space around the exhaust muffler 11 to cool the exhaust muffler 11 and is discharged outside together with the exhaust gas. Therefore, it is difficult to decrease the temperature of the exhaust gas discharged through the exhaust gas discharge opening 110.
Flanges 115 forming caulked joints and protruding from the outer surface of the exhaust muffler 11 impede the flow of the cooling air flowing along the upper surface of the exhaust muffler 11.